1. Field of the Invention
The present invention relates to an apparatus generating an optical pulse.
2. Description of the Related Art
An optical communications system currently adopts a wavelength division multiplexing (WDM) method implementing a communications bit rate of 10 giga bits/s (Gb/s, giga: 109) or 40 Gb/s. With this method, however, the number of wavelengths becomes greater with an increase in a communications capacity. Therefore, it is difficult to manage signals having respective wavelengths. Additionally, a system that synchronizes signal light beams having different wavelengths, and executes signal processing in real time is required. Its configuration becomes very complex. From such viewpoints, an optical time division multiplexing (OTDM) method that enables a mass-capacity communication is considered promising. For example, a communications light source used in the OTDM must stably generate an optical pulse that is accurate to 10 Gb/s and has a pulse width of picosecond (ps, pico: 10−12) class, if a 10-Gb/s signal is multiplexed to 160 Gb/s. That is, the optical pulse having a time width, which is sufficiently shorter than the cycle time of bit rate, namely, a time width having a high duty ratio, must be stably generated at an accurate bit rate.
For this implementation, conventional techniques for generating an optical pulse are broadly classified into the following two types.    (1) a technique using a mode-locked laser pulse light source    (2) a technique using a direct modulation pulse light source
FIGS. 1A and 1B show the basic configurations of the techniques using a mode-locked laser pulse light source.
Specific examples include a semiconductor mode-locked laser (FIG. 1A), and a fiber mode-locked laser (FIG. 1B). By controlling parameters such as the frequency of a driving RF (Radio Frequency) signal, a phase, the power of laser light for pumping a gain, etc., an optical pulse of subpicosecond class, which has a high optical signal-to-noise ratio, is not restricted by the frequency of the RF signal, and has a high duty ratio, can be generated. However, there is a problem that the mode-locked laser pulse light sources cannot arbitrarily and accurately implement the repetition frequency of an optical pulse due to their structures. In the mode-locked lasers, the repetition frequency f0 of an optical pulse must satisfy the following equation in a relationship with the length of a resonator of the lasers if it is assumed that the velocity of light is c, the refractive index of a resonator medium is n, and N is an arbitrary integer.
                              f          0                =                  N          ⁢                      c                          2              ⁢              nL                                                          (        1        )            
Accordingly, to generate an optical pulse of a certain accurate repetition frequency f0, for example, 10 giga hertz (GHz) ±100 Hz, L must be accurately manufactured.
By way of example, for the semiconductor mode-locked laser, the length of its resonator is on the order of 1 centimeter (cm), and its error margin must be suppressed to 1 nanometer (nano: 10−9) or smaller. Its implementation is difficult if its yield in commercialization is considered. In the meantime, for the fiber mode-locked laser, the length of its resonator is on the order of several tens of meters. Accordingly, only N is adjusted, and there is no need to adjust the length as strictly as in the semiconductor mode-locked laser. However, since the length of the resonator is long, it significantly varies with a small temperature change, etc. Therefore, it is difficult to stably operate the laser at an arbitrary and accurate repetition frequency.
FIG. 2 shows the basic configuration of the technique using a direct modulation pulse light source.
Its specific examples include a pulse light source using an electro-absorption modulator (EAM). This light source is configured by a single wavelength laser light source, an EAM, an RF signal source for driving the EAM, and a direct current voltage source. With this method, a stable optical pulse can be accurately generated at an arbitrary repetition frequency according to the control of the RF signal source for driving the EAM. However, since the optical transmission loss of the EAM is large, the optical signal-to-noise ratio (OSNR) of generated pulse light is significantly deteriorated when output power is amplified with an optical amplifier. For example, the optical transmission loss becomes 20 decibels (dB) or more because a reverse bias DC voltage is applied to the EAM by the direct current voltage source when an optical pulse is generated, although the optical transmission loss of the EAM itself is approximately 7 dB. This leads to the deterioration of the OSNR. Additionally, it is difficult to generate an optical pulse having a high ratio (duty ratio) of the cycle time of a modulation frequency to the time width of the optical pulse, since the waveform of the generated optical pulse depends on that of the RF signal source for driving the EAM.
As a reference document of the above described technique, Non-Patent Document 1 exists.
[Non-Patent Document 1]
IEEE Journal of Quantum Electronics, Vol. 24, No. February 1988, pp. 382–387, title “Optical Pulse Compression Using High-Frequency; Electrooptic Phase Modulation”
As described above, the mode-locked laser pulse light source can stably generate a subpicosecond class optical pulse having a high OSNR. However, it is difficult to manufacture a mode-locked laser pulse light source having an accurate length of a resonator also from a yield viewpoint, and to implement stable operations at an arbitrary and accurate repetition frequency. In the meantime, for the direct modulation pulse light source, its control is easy, and an optical pulse at an arbitrary and accurate repetition frequency can be generated. However, there are disadvantages such that: (1) an optical transmission loss in an optical intensity modulator is large, leading to the deterioration of the OSNR, and (2) it is difficult to generate an optical pulse having a high duty ratio since the width of the pulse is restricted by a modulation frequency.